Despite being shown in the First Church of Springfield, Lenny is a Buddhist — making him, Carl, and Lisa the only three Buddhists in Springfield.[1] Born in Chicago, he is also a war hero and a three-time juror. His grandmother spent 20 years in a Soviet labor camp, hinting that Lenny may have ancestors from the former Soviet Union, or have ancestors who were POWs.[2] During one episode, he is shown to be an adept guitar player.[3] "Homer the Great" reveals Lenny was member number 12 of the Springfield chapter of the Stonecutters secret society. In the episode "Half-Decent Proposal", Lenny is shown to have little regard for his own life: "Quick and pointless, that's the death for me." Another example of his apathy towards salvation is that he shrugs after being pulled from a ladder which would have seen him and Homer to safety. Excluding unnamed background characters, Lenny is one of the few Springfield residents to persistently resist the lure of the Movementarians. Lenny is a member of the Republican Party, as indicated in "Bart-Mangled Banner" (2004) when he reveals his Dole/Kemp '96 American flag tattoo. He is also shown to be a member of the NRA as seen in "The Cartridge Family". His weapon of choice appears to be an AR-15 rifle, which he says "are manufactured for a reason: to take out today's modern super animals like the flying squirrel and the electric eel." [4]

Lenny appears to be well liked by the Simpson family – on one occasion, Marge and the kids build a prayer shrine for him when learning he was taken to the hospital. In "Hello Gutter, Hello Fadder", Homer goes bowling but tells Marge that Lenny was hospitalized; when Marge informs the kids, they respond the same way she did and shout, "Not Lenny!". In "Sleeping with the Enemy", the Simpson family has a cake inscribed "Happy Labor Day Lenny".[10] In "Pranksta Rap", it is revealed Marge has a picture of Lenny in her hair.[11]

Lenny is apparently divorced – in "Bart of War", Carl says that he sang "The Best Is Yet to Come" at Lenny's wedding,[10] although in another episode Lenny punches Carl for giving a bad speech at his wedding. Lenny is a persistent bachelor who has poor luck with women. In "Marge on the Lam", Lenny shaves the legs of a woman who calls him an idiot for not shaving in an upward direction.[12] In "Team Homer", Homer mentions to Moe that both Lenny and Carl are with their mistresses. In "Homer the Smithers" he is seen in the opening scene with his arm around a woman who appears to be his girlfriend.[13] In "Homer Simpson, This Is Your Wife", it's revealed that he had dated a woman in a Woody Woodpecker outfit at a fairground for three months until she left him for the man who cleans the vomit on the roller coaster.[14] Lenny had told Carl that he was married to a beauty queen, and as of the 8th season, Carl had yet to meet Lenny's wife. The episode "In Marge We Trust", it is revealed that Lenny made his Beauty-Queen wife up, and is unmarried.

Lenny's full name has not been treated with consistency. After years of being identified only as "Lenny", Homer addressed him as "Lenford" in the season 13 episode "The Frying Game", and Lisa addressed him as "Mr. Leonard" in the season 15 episode "The Ziff Who Came To Dinner". This would appear to make his full name "Lenford Leonard" -- but Bart addresses him as "Lenny Lenford" in the season 23 episode "At Long Last Leave."

Carl is voiced by Hank Azaria. Along with Lenny, Carl is Homer's friend and co-worker (sometimes identified as his supervisor) at the Nuclear Power Plant. He likes to call himself "an urban Lenny." He attended Springfield A&M University.

In the early seasons, Carl was rarely seen with Lenny and did not have a consistent voice – on some occasions, he can be heard with Lenny's voice and vice versa. One example of Carl having Lenny's voice is in "Brush with Greatness". In an early 1991 episode, "Principal Charming", Carl's name is spelled "Karl", the same spelling seen for an unrelated character of the same season.

In the fourteenth season episode 'Scuse Me While I Miss the Sky, Carl made reference to his childhood in Iceland which hinted that he may have Icelandic ancestry. In the twenty-fourth season episode The Saga of Carl it was in fact revealed that he was adopted by a couple in Iceland. Carl often serves as a taciturn straight man to Homer and company, because of his harsh opinions of others,[citation needed] as seen in "Pygmoelian" when he makes everyone in Moe's Tavern cry and, breaking the fourth wall, explains: "See, this is why I don't talk much." He is offended when people stereotype him even though their stereotypes are usually correct. In "The Great Wife Hope", Homer asks Carl if he knows African-American boxer Dredrick Tatum; Carl takes offense, saying that just because he is African American, he doesn't know all the other African-American citizens in Springfield. However he then says that he indeed met Dredrick Tatum while he was at a party with Dr. Hibbert at Bleeding Gums Murphy's house (both of those characters are also African-American). More notably, in Children of a Lesser Clod, during a basketball game, Carl voices his irritation when everybody makes high expectations of his performance, and he remarks that he isn't a good player only because he is an African American, but proves himself, however, to be an amazing basketball player. Despite being against racism, Carl has some sort of dislike of Muslims, visible in Mypods and Boomsticks. He is the intellectual one, but apparently, Lenny, his best friend, dumbs him down for his own sake. In the episode "The Girl Who Slept Too Little", he tells Lenny to shut up as he was tired of "dumbing [him]self down for [Lenny].". Despite being a good friend, Carl can be very self-centered and workaholic at times – in The Devil Wears Nada, Carl becomes the new supervisor of Homer's sector (for being the most competent of them), and starts to abuse his position by forcing Homer to become his assistant, making him do even the mostly simple and unnecessary tasks (much to Homer's dismay).

During a conversation in the Springfield Hub, Carl and Bernice Hibbert are hinted to have had a past relationship, that Carl wants to rekindle. It is implied, however, that he has a wife in "Sunday, Cruddy Sunday", when he says he will give a ring to his wife for their anniversary. Also, he is shown with a woman (presumably his wife) attending Homer's marriage class in "Secrets of a Successful Marriage". He also mentions having a family in "HOMR". In "Lisa the Drama Queen", he appears in a car with Lenny and two women, possibly one is his wife.

Lenny and Carl are best friends, as they are rarely seen apart; their other friends are Homer, and regulars at Moe's including Barney Gumble and Moe Szyslak.[15] Homer repeatedly confuses Lenny and Carl, and is shocked to learn on one occasion that Lenny is white, and Carl is black. To guide himself, Homer has "Lenny = White, Carl = Black" on his hand. He once muttered to himself, "Is that right?" while reading it.[16] In "Helter Shelter", Homer exclaims, "That's Lenny? I wanted the black one!" When Mr. Burns appears on a radio show in an attempt to boost his popularity in "Monty Can't Buy Me Love", Homer tells him that he has a list of jokes explaining the differences between white and black people; Homer later states, "White guys have names like Lenny, whereas black guys have names like Carl."

There are occasional homosexual overtones in Lenny's and Carl's relationship. In another episode, "'Scuse Me While I Miss the Sky", various characters look at the stars to "see into their souls". Lenny sees Carl's face in the sky, who also sees his own face.[17] At times, Carl gets annoyed at Lenny. In "Mountain of Madness", when Carl learns that he is paired with Lenny in a team-building exercise on a power plant retreat, his irritated response is, "Aw, nuts. I mean... aw, nuts." In the Simpsons Game, Carl and Lenny are shown arguing while bird-watching.

In the non-canon episode "Behind the Laughter", Lenny and Carl are constant companions, and Bart had apparently paid the two to kiss one another.[18] A running gag through the series is the possibility of a sexual relationship between Lenny and Carl. When Marge's Popsicle stick sculptures of Lenny and Carl are destroyed and mashed together, Lenny states, "I don't know where Carl ends and I begin!", to which Carl replies, "See, statements like that are why people think we're gay".[19]

Carl tells Seymour Skinner that "Marriage is gonna be great. Now you'll have someone who'll rub your back – without being asked" while glaring at Lenny, who sighs "Oh, not this again." Carl then responds "Yes, this again."[20] In one episode, Homer runs out of gay couples to marry; he speculates that Lenny and Carl might be interested, and Marge responds with "Don't you push them – they've gotta work that out for themselves."[21] In "Treehouse of Horror XVI", Lenny dies and sees the angels in Heaven take the form of Carl (although they're saying, "Hurry up or we'll be late for work at the plant!").[22] Once, Chief Wiggum remarks that Lenny's relationship with Carl is on the rocks.[23] In "Fat Man and Little Boy", Homer quotes to Lisa "You and science go together like Lenny and Carl – the science is Carl".[24] In "Marge and Homer Turn a Couple Play", Lenny and Carl see Homer and Marge kiss, prompting Lenny to remark, "Remember when we used to kiss like that . . . with our respective girlfriends?" [25]

Lenny's hero worship of Carl has reached a mounting point. In "Half-Decent Proposal", it's shown he carved Carl's face on a mountain, and called it Mount Carlmore. He later leaves a burning oil field, only because Carl arrived.[26] Lenny publishes a newspaper called The Lenny-Saver with the headline: "The Truth About Carl: He's Great." After displaying this, he sheds a tear and states that "it had to be told."[27] In the episode "Lisa the Drama Queen", Lenny and Carl are pictured driving a sports car through the woods with their two dates in the back of contrasting background.

During "Whiskey Business" in the 24th season Carl and Lenny hold hands together for a long moment and then release while standing together through a limomoonroof.[28]

Lenny and Carl work at the Springfield Nuclear Power Plant alongside Homer Simpson.[29] Despite his steady job, Lenny has been featured with several other jobs. On one occasion, he is promoted to head of the power plant when Mr. Burns goes bankrupt; which Smithers later describes to Homer Simpson as a "reign of terror". Homer considers Lenny to be the second richest man he knows.[30] However, Lenny is once shown living in a dilapidated house, and asks Marge to not tell anyone how he lives.[31] In one episode, he also works at a call center for the power company in Springfield. In another episode, he is shown living in a fancy, well-furnished modern apartment that happens to share a wall with a jai-alai court. In a "future episode", it's shown Lenny is once again in charge of the power plant.[32] On one occasion, it is implied he is an undercover agent whose target is Homer.[33] At the Adult Education Annex, Lenny teaches a class on "How To Chew Tobacco".[34] In one of Homer's daydreams, it's shown that Lenny is the President of the United States.[35] Lenny is also a successful writer who wrote a series of mystery novels, one of them entitled The Murderer Did It, which are called "scary good fun" by Stephen King. Lenny has a short career as an actor in which he has a minor role in a horror movie that Homer takes Bart and Lisa to. His acting is shown to be a parody towards bad acting in horror movies.